Method and apparatus for controlling the voltage of signals powering low voltage lighting systems

ABSTRACT

A power control system and method of power control, the system consists of a power transformer having a primary portion and a secondary portion and a wiring run having a length and coupled at one end to the secondary portion. Also included is a constant voltage controller coupled to another end of the wiring run. The constant voltage controller receives an input power signal from the power transformer and outputs an output power signal at a specified voltage level to a plurality of low voltage electrical loads. The input power signal is at a higher voltage level than the specified voltage level and the output power signal is maintained at the specified voltage level regardless of a change in a voltage drop of the power control system. In preferred embodiments, the system uses low power logic rectifiers to rectify AC power signals to provide reset, current feedback and voltage feedback signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/715,930, filed Nov. 17, 2000, now U.S. Pat. No. 6,426,611 (AttorneyDocket 68549), of John R. Reeves et al., for METHOD AND APPARATUS FORCONTROLLING THE VOLTAGE OF SIGNALS POWERING LOW VOLTAGE LIGHTINGSYSTEMS, which application is hereby fully incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power control circuits, and morespecifically to power control circuits for low voltage lighting systems.Even more specifically, the present invention relates to power controlcircuits for low voltage lighting systems having variable loads.

2. Discussion of the Related Art

Low voltage lighting systems, such as found in outdoor lighting systems,typically include electrical loads that use a lower voltage than thestandard house current circuit that provides 120 volts AC. Such lowvoltage lights commonly require a 12 volt AC input.

A transformer is used to convert power at 120 volts AC to power at 12volts AC that is used for the input at the electrical loads, e.g. thelamps. Typically, in outdoor lighting applications, the lamps arecoupled to the transformer via parallel wiring and are located atvarying distances from the transformer. The wiring supporting a stringof lamps is referred to as a wiring run or a lighting run.Disadvantageously, a lamp load that is electrically remote from thetransformer will burn more dimly than an identical lamp loadelectrically close to the transformer. This results from the resistiveloss and consequent voltage drop in the wiring from the transformer tothe lamp load. The longer the distance from the transformer, the largerthe voltage drop across the wires. Thus, the effective voltage at thelamps may be less than 12 volts AC.

Furthermore, low voltage lighting systems are designed to be flexible inthe number and positioning of lamp loads with respect to distance alongpower distribution wires. However, by changing the number of lamp loadspresent in the system, the total voltage drop is varied, resulting in anoverall brightness change in the remaining lamp loads. For example, anincrease in lamp loads with the same power signal, results in a lowervoltage at the lamp loads such that each lamp will appear to dim.Additionally, if one lamp load blows, the voltage in the remaining lampswill increase, leading to the remaining lamps running brighter. Theincreased voltage in the remaining lamps may contribute to theirpremature failure.

One solution is to distribute power at a high voltage, but locallyreduce the voltage for each lamp load. This results in running a highvoltage signal from the standard household current circuit to the lamploads, leading to increased risk of electrical shock in the event of aperson accidentally coming into contact with a the wiring carrying thehigh voltage power signal. Disadvantageously, such a solution alsorequires a separate transformer at each lamp load to convert the highvoltage power signal to a voltage level useable by the respective lampload.

Another solution is to control the voltage applied to the primarywindings of the transformer. Typically, sensing wires are used to sensethe voltage applied to the lamp loads and provide feedback to acontroller coupled to the primary windings of the transformer. Thiscontroller controls the voltage applied to the primary windings whichcontrols the output voltage of the secondary circuit such that thevoltage at the lamp loads is at the desired level. Thus, the outputvoltage is greater than 12 volts AC, but is about 12 volts AC after thevoltage drop across the wires. This approach uses triacs at the primarywindings and disadvantageously requires additional sensing wires to runfrom the remotely located electrical loads back to the transformer.Furthermore, since each lighting run will have a respective voltage dropassociated with it, each lighting run requires a separate transformer toconvert the 120 volt AC power signal to a voltage level that once thevoltage drop of the specific lighting run is accounted for, will beabout 12 volts AC. Each of the separate transformers is controlled byits own triac in the primary and has its own sensing wires, whichsignificantly add to the cost of the system.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing a constant voltage controller coupled to andlocated remotely from the secondary of a power transformer formaintaining the voltage at a plurality of electrical loads at a constantlevel regardless of the voltage drop associated with powering theplurality of electrical loads.

In one embodiment, the invention can be characterized as a power controlsystem including a power transformer having a primary portion and asecondary portion and a wiring run having a length and coupled at oneend to the secondary portion. Also included is a constant voltagecontroller coupled to another end of the wiring run. The constantvoltage controller receives an input power signal from the powertransformer and outputs an output power signal at a specified voltagelevel to a plurality of low voltage electrical loads. The input powersignal is at a higher voltage level than the specified voltage level andthe output power signal is maintained at the specified voltage levelregardless of a change in a voltage drop of the power control system.

In another embodiment, the invention can be characterized as a constantvoltage controller for maintaining a power signal at a constant voltageat a plurality of electrical loads including a switch receiving an inputAC power signal from a secondary portion of a power transformer andoutputting an output AC power signal to the plurality of electricalloads. Also included are a controller coupled to the switch and a firstlogic rectifier circuit coupled to the controller for inputting theoutput AC power signal and a return AC power signal and outputting avoltage feedback signal to the controller. A second logic rectifiercircuit is coupled to the controller and inputs the output AC powersignal and outputs a current feedback signal to the controller. And, areset controller including a third logic rectifier circuit is coupled tothe controller for inputting the input AC power signal and outputting areset signal to the controller. The controller maintains a voltage levelof the output AC power signal at a specified level by controlling theoperation of the switch based upon the reset signal, the currentfeedback signal and the voltage feedback signal.

In a further embodiment, the invention can be characterized as a methodof controlling the voltage of a power signal for a plurality ofelectrical loads comprising the steps of: receiving an AC power signalfrom a secondary portion of a power transformer, wherein the AC powersignal is at a voltage greater than a specified level corresponding tothe plurality of electrical loads; adjusting the voltage of the AC powersignal to a voltage level such that the voltage supplied to theplurality of electrical loads is at the specified level; and maintainingthe voltage of the AC power signal at the specified level regardless ofa change in a voltage drop experienced by the AC power signal.

In yet another embodiment, the invention can be characterized as avoltage feedback for a power control system for outdoor lighting systemsincluding a switch inputting an input AC power signal and for providingan AC output power signal to a plurality of lamp loads and a controllercoupled to the switch and controlling the operation of the switch. Alsoincluded is a logic rectifier circuit coupled to the controller whereinthe logic rectifier circuit inputs the AC output power signal and areturn AC power signal from the plurality of lamp loads. The logicrectifier circuit provides a rectified voltage feedback signal for thecontroller which is used to adjust the operation of the switch.

In yet another further embodiment, the invention can be characterized asa current feedback for a power control system for outdoor lightingsystems including a switch inputting an input AC power signal and forproviding an AC output power signal to a plurality of lamp loads and acontroller coupled to the switch for controlling the operation of theswitch. Also included is a logic rectifier circuit coupled to thecontroller wherein the logic rectifier circuit inputs the AC outputpower signal across a current sense resistor. The logic rectifiercircuit provides a rectified current feedback signal for the controllerwhich is used to adjust the operation of the switch.

In a subsequent embodiment, the invention can be characterized as apower controller for outdoor lighting systems including a switchinputting an input AC power signal and for providing an AC output powersignal to a plurality of lamp loads and a controller coupled to theswitch for controlling the operation of the switch. Also included is alogic rectifier circuit coupled to the controller wherein the logicrectifier circuit inputs the AC input power signal and outputs a resetsignal for the controller. The reset signal comprises a pulse signalsent to the controller at the beginning of each half cycle of the inputAC power signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of a power supply system including a constantvoltage controller located remotely from a power supply transformer andfor controlling, in the secondary side, the voltage to the lamp loads inaccordance with one embodiment of the invention;

FIG. 2 is a variation of the power supply system of FIG. 1 employing amultiple tap power supply transformer such that the power signal outputto the constant voltage controller is selectable in accordance withanother embodiment of the invention;

FIG. 3 is a functional block diagram of one embodiment of the constantvoltage controller of FIG. 2;

FIG. 4 is a functional block diagram of another embodiment of theconstant voltage controller of FIG. 2;

FIGS. 5, 5A and 5B are a schematic diagram of the constant voltagecontroller of FIG. 3 in accordance with an embodiment of the invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

In one embodiment, a power control circuit for outdoor lighting systemsis provided that uses low loss power field effect transistors and logiclevel control circuits to transform AC currents and voltages into usableDC signals to operate a sophisticated phase control circuit. Thisprovides a low cost, low loss, and efficient phase control in thesecondary of a power transformer to be used in outdoor lighting systemsusing approximately 12 volts at currents between 20 and 90 amps.

Referring first to FIG. 1, a block diagram is shown of a power supplysystem including a constant voltage controller located remotely from apower supply transformer and for controlling, in the primary side of thepower transformer, the voltage to the lamp loads in accordance with oneembodiment of the invention. Shown is a power supply system 100including supply power 102, power transformer 104 (also referred to as atransformer 104) having a primary side 106 and a secondary side 108,wiring run 110, constant voltage controller 112 and lamps 114, 116, and118 (also referred to generically as electrical loads 114, 116, and 118)electrically coupled in parallel to the constant voltage controller 112.

As is common to outdoor lighting systems, it is preferable to distributepower to low voltage electrical loads, such as lamps 114, 116 and 118 ata lower voltage than the standard household power supply, which is shownas supply power 102 and is a current of 120 volts at 60 hertz. This isdue to the fact that most lighting systems have lamps that operate atconsiderably lower voltages than the supply power at 120 volts AC. Forexample, most outdoor lighting systems have lamps that operate at 12volts for safety electrical low voltage (SELV). As such, a powertransformer steps down the voltage from the 120 volts AC signal to a 12volt AC signal. The power is distributed to the lamps 114, 116 and 118at the lower voltage to advantageously reduce the risk of electricalshock and to avoid the use of separate power transformers located ateach set of lamps.

As described above, the lamps 114, 116, and 118 are typically located atthe end of a wiring run 110 that is remotely located from the powertransformer 104. This wiring run 110 has a voltage drop due toresistance such that the voltage signal at the lamps 114, 116 and 118may actually be less than 12 volts AC. This lowered voltage results inthe lamps illuminating less brightly. Additionally, the further locatedthe lamps 114, 116 and 118 are from the power transformer 104, thehigher the voltage drop across the wiring run 110, again affecting thebrightness of the lamps 114, 116 and 118. Furthermore, by simply addingor subtracting the number of lamps coupled to a wiring run 110, thetotal voltage drop is altered, again, effecting the overall brightnessof the lamps 114, 116, and 118. For example, the addition of one lampwill increase the voltage drop, and thus, all of the lamps will burnless brightly.

Some conventional approaches distribute the power at a higher voltageand then locally convert the voltage to the required low voltage signal.However, such approaches require a separate transformer at the end ofeach wiring run. Such transformers are bulky, expensive and difficult tohide in an outdoor environment. This approach is very hazardous due torunning the high voltage line to the lamps, e.g., through yards andgardens, which leads to the increased risk of electrical shock.Furthermore, depending on the number of lamps, there is still a variableamount of voltage drop.

Other conventional approaches control the voltage at the primary side106 of the power transformer 104, for example, with a controller coupledto a triac, such that the power signal produced at the secondary side108 of the transformer 104 is adjusted to create the desired voltage atthe lamps 114, 116, and 118 accounting for the voltage drop across thewiring run. Additional sensing wires can be used to feedback the voltageat the lamps 114, 116 and 118 to a controller coupled to the triad atthe primary side 106 of the transformer 104. Additionally, a separatetransformer is required for each wiring run, since each wiring run willhave its own voltage drop associated therewith.

In contrast, and accordance with an embodiment of the invention, thepower supply system 100 provides a transformer 104 capable of providinga power signal at the secondary side 108 that has a greater voltage thanthe voltage required by the lamps 114, 116 and 118. For example, thetransformer 104 converts the 120 volt AC signal to a 20 volt AC signal.As such, given the voltage drop across the length of the wiring run 110,the voltage at the lamps 114, 116, and 118 is not less than the desiredvoltage of the electrical loads, e.g. 12 volts. Also, advantageously,the power signal coupled to the lamps is at a lower, safer voltage levelthan the 120 volt AC power supply 102.

Additionally, a constant voltage controller 112 is coupled to the end ofthe wiring run 110. The lamps 114, 116 and 118 are coupled to theconstant voltage controller 112. The constant voltage controller 112inputs the power signal from the secondary side 108 of the transformer104 and outputs a power signal to the lamps 114, 116, and 118 that isapproximately the desired voltage, e.g. 12 volts AC, using a phasecontrol circuit in one embodiment. The constant voltage controller 112acts as a hub for the lamps 114, 116 and 118 and reduces the amount ofwiring necessary for the lamps, since all lamps 114, 116, and 118 arecoupled directly to the constant voltage controller 112, not coupled tothe transformer 104 itself.

Advantageously, the constant voltage controller 112 utilizes a low losspower switch, such as a power field effect transistor (FET), to switchthe 20 volt AC signal to a 12 volt AC signal. A phase controller withinthe constant voltage controller 112 delays the point in the phase beforethe low loss power switch turns on, similar to triac control. Such FETpower switches typically have a forward voltage drop of about 0.2 volts;thus, with a 20 amp current in the secondary side 108, the FETdissipates only 4 watts.

Alternatively, a triac based controller may be implemented in thesecondary side 108 of the transformer 104, similar to that employed inconventional systems in the primary side 106. However, such triac basedcontrollers in the secondary windings are not employed, since thecurrent output from the secondary side 108 is about 10 times greaterthan the current at the primary side 106. Due to the voltage drop acrossthe triac and the increased current in the secondary side 108, therewill be considerably more power loss than if the triac was on theprimary side 106. A triac with about a 1.4 volt forward drop and woulddisadvantageously dissipate about 28 watts of power (assuming a 20 ampcurrent in the secondary side, according to Watt's law, i.e. P=VI).Thus, triacs coupled to the secondary side 108 of the transformer 104would require space-consuming heat sinks and the fixtures containingsuch triacs would get very hot.

Furthermore, the constant voltage controller 112 utilizes low power/lowcost semiconductor logic to sense, full wave rectify, and process thepower signals. Thus, the input power signal to the constant voltagecontroller 112 is rectified with a logic rectifier circuit, e.g., an opamp rectifier, to provide a reset signal for the phase controller, theoutput power signal is rectified with a logic rectifier circuit, e.g., afull wave op amp rectifier, to provide a current sense signal, and theoutput signal to the lamps and the return signal from the lamps isrectified with a logic rectifier circuit, e.g., a full wave op amprectifier, to provide a DC voltage sense signal that can easily be usedby the phase controller.

Thus, advantageously, the power signal to the lamps 114, 116 and 118 isan AC power signal, not a DC signal. As such, the power signal to thelamps is not required to be rectified to DC. A lamp controller could beeasily built such that the AC power signal from the secondary side 108is rectified to DC by a traditional full wave bridge rectifier, forexample. As is known, the diodes of such a rectifier would drop about 2volts resulting in a 40 watt loss with a 20 amp current in the powersignal. Additionally, a silicon controlled rectifier (SCR) would be usedto output the power signal to the lamps. There would also be additionalpower loss from the SCR. Such a circuit would be easy to build since allof the signals are in DC; however, the circuit would suffer from severepower loss and thus impractical.

In accordance with one embodiment, the voltage and current feedbacksignals are converted to DC using logic rectifier circuits, e.g., op amprectifiers. These op amp rectifiers only lose about 0.5 watts of power.In another approach in which the power signal remains AC, the currentand voltage feedback signals must be rectified to DC. Such an approachwould employ current transformers and optocoupler drives to convertthese feedback signals to DC. However, current transformers andoptocouplers are expensive as well as suffer from power losses.Additionally, as described above, a triac would be used to control thevoltage of the power signal to the lamps. Disadvantageously, asdiscussed above, the triac dissipates about 28 watts in the secondarygiven about 20 amps of current. In contrast, this embodiment employs alow loss FET power switch instead of a triac. Furthermore, thisembodiment does not require additional sensing wires from the lamps 114,116 and 118 since the constant voltage controller 112 is used. Furtherdetails of embodiments of the constant voltage controller 112 aredescribed with reference to FIGS. 3 through 5.

Additionally, the constant voltage controller 112 provides a powersignal to the lamps 114, 116, and 118 such that the power signaldelivered to the lamps 114, 116, and 118 is maintained at the desiredlevel, for example, 12 volts, regardless of the voltage drop in thewiring run 110 or the supply power from the transformer 104. Theconstant voltage controller 112 compensates for any voltage drop fromthe constant voltage controller 112 to the lamps. Furthermore, theconstant voltage controller 112 adjusts the output power signal to thelamps 114, 116 and 118, in the event the number of lamps is changed. Forexample, if more lamps are coupled to the constant voltage controller112, the constant voltage controller 112 senses the additional voltagedrop and adjusts the output power signal to the lamps such that itremains at the desired level. Also, advantageously, if any lamps burnout, the voltage to the remaining lamps is adjusted so that it remainsat the desired voltage level, e.g., 12 volts AC. Accordingly, all of thelamps coupled to the constant voltage controller 112 will have the sameinput power signal at the desired voltage regardless of the number ofthe lamps or a change in the number of lamps. In operation, a change inthe number of lamps will not effect the brightness of the lamps coupledto the constant voltage controller 112.

Due to the modular nature of the system, multiple wiring runs 110 may becoupled to the power transformer 104, each wiring run 110 having aseparate constant voltage controller 112 at its end and including aplurality of electrical loads (e.g. lamps) coupled in parallel to therespective constant voltage controller 112.

Additionally, with a constant voltage controller 112 coupled to the endof multiple wiring runs 110, the lamps at the end of each of the wiringruns 110 (i.e. the lamps coupled to each constant voltage controller112) will all have a power signal at approximately the desired operatingvoltage. Thus, lamps coupled to adjacent wiring runs 110 will have thesame brightness level that is unaffected by the distance from thetransformer 104 and by a change in the number of lamps.

It is noted that although only one constant voltage controller 112 isillustrated as being coupled to the secondary side 108 of the powertransformer 104, that the skilled artist could easily attach severalconstant voltage controllers 112 to the secondary, each with multiplelamps 114, 116 and 118 coupled to the respective constant voltagecontrollers 112, similar to coupling multiple lamps directly to thesecondary of the transformer.

Referring next to FIG. 2, a variation of the power supply system 100 ofFIG. 1 is shown employing a multiple tap power transformer 202 such thatthe power signal output to the constant voltage controller 112 isselectable in accordance with another embodiment of the invention. Thepower supply system 200 of FIG. 2 has the same components as the powersupply system 100 of FIG. 1 except the power supply system 200 of FIG. 2uses a multiple tap power transformer 202 in place of power transformer104. This multiple tap power transformer 202 provides multiple taps 206,208, 210, 212, and 214 in the secondary side 204 that provide aselectable voltage level in the output power signal. For example, themultiple tap power transformer 202 provides power signals at 12 voltsAC, 14 volts AC, 16 volts AC, 18 volts AC and 20 volts AC (e.g. at taps206, 208, 210, 212, and 214). Such a multiple tap power transformer 202is commercially available from Unique Lighting Systems of Escondido,Calif., as a “Multimatic Power Transformer”, e.g., Part No. T-840SS.Other suitable transformers available from Unique Lighting Systemsinclude Multimatic Power Transformers Part Nos. T-300SS, T-360SS,T-500SS, T-600SS and T-1120SS. Advantageously, the lighting designer canchoose which of the multiple taps to attach a wiring run 110 andconstant voltage controller 112, depending on the desired voltage of theelectrical loads 114, 116 and 118, the voltage drop across the wiringrun 110, and the number of electrical loads to be coupled to theconstant voltage controller 112. Generally, in several embodiments ofthe invention, it is desirable to have the voltage of the power signaloutput from the multiple tap power transformer 202 to be greater thanthe operating voltage of the electrical loads so that the constantvoltage controller 112 can appropriately adjust and maintain the powersignal to the electrical loads at the desired level. Furthermore,whether using the multiple tap power transformer 202 of FIG. 2 or thepower transformer 104 of FIG. 1, it is important to note that thecurrent in the secondary side 108 and 208 is relatively high, forexample, greater than 15 amps, e.g., 20 amps, 50 amps, 90 amps, etc.This is important because it effects the power loss in devices that thecurrent passes through. For example, the higher the voltage drop acrossa device, the higher the power loss.

Referring next to FIG. 3, a functional block diagram is shown of oneembodiment of the constant voltage controller of FIG. 2 employing aphase controller. Shown is the constant voltage controller 112 includinga power switch 302 (also referred to generically as a switch 302), areset controller 304, a first logic rectifier circuit 306, a secondlogic rectifier circuit 308, and a phase controller 310. Alsoillustrated are the input power signal 312, a reset signal 314, anoutput power signal 316, a current feedback signal 318, a lamp returnsignal 320, a voltage feedback signal 322, a timer signal 324, and areturn signal 326. Also illustrated are lamps 114, 116 and 118.

The input power signal 312 is received from the power transformer ofpower supply system, e.g. power transformer 104. In this example, theinput power signal 312 is at approximately 20 volts AC, although may beless due to the voltage drop in the wiring used to distribute the powersignal to the constant voltage controller 112. However, in thisembodiment of the invention, it is important that the input power signal312 be at a voltage that is greater than the operating voltage of theelectrical load/s, e.g. greater than 12 volts AC. The input power signal312 is input into the constant voltage controller 112 which operates tokeep the voltage to the lamps constant by delaying the point in thephase before the power switch 302 turns on, similar to triac control.

The input power signal 312 is fed into the reset controller 304 which isa logic rectifier circuit, e.g., a full wave op amp rectifier, in orderto output a reset signal 314 to the phase controller 310. This resetsignal 314 is illustrated as a DC pulse signal 328 that is sent at thebeginning of each half cycle of the input power signal 312. Thus, theinput power signal 312 is rectified to DC and referenced to logiccommon. This is a departure from the prior art in that phase controlcircuits in the primary side which may be used for triac control in theprimary use full wave bridge rectifiers which use 4 diodes per phase toprovide such signals as opposed to logic rectifier circuits. If employedin the secondary side, such full wave bridge rectifiers, for example,which have a 2.0 volt drop, would result in a 40 watt power loss given acurrent in the secondary of 20 amps. In contrast, the use of low poweranalog logic, such as op amp type rectifiers, results in much lowerpower loss than full wave bridge rectifiers (e.g. about 0.5 watts powerloss compared to about 40 watts for the full wave bridge rectifier).Note that in other embodiments, the current in the AC power signal 312is greater than 20 amps, e.g., 50 amps or 90 amps, such that there is aneven greater power loss from the use a full wave bridge rectifier incomparison to an op amp type rectifier. Thus, op amp rectifiers requirevery little DC power and dissipate very little power in comparison tofull wave bridge rectifiers. Full wave op amp rectifiers are well knownand understood by those skilled in the art. Note that the resetcontroller 304 also inputs the return signal 326.

The phase controller 310 is a logic circuit which is used as a timer forthe low loss switch 302. This phase control circuit is similar to suchcontrol circuits used to control a triac. The phase controller 130inputs the reset signal 314 and delays the point in the phase before theswitch 302 turns on via the timer signal 324; thus, reducing the voltageof the output signal 316 to a desired level. The longer the delay, thelower the voltage applied to the lamps in the output power signal 316.The input power signal 312 is an AC signal which is illustrated as ACwaveform 330, while the phase delayed output power signal 316 isillustrated as AC waveform 332.

The timer delay of the phase controller 310 utilizes a current sensesignal 318 which provides a measure of the current of the output powersignal 316 and a voltage sense signal 322 that provides a measure of thevoltage as applied to the lamps in order to control the exact point atwhich the switch 302 is turned on. Advantageously, a first logicrectifier circuit 306, e.g., a full wave op amp rectifier, rectifies thelamp return signal 320 and the output power signal 316. This provides aDC value used to estimate the voltage actually applied to the lamps 114,116 and 118. The phase controller 310 uses the voltage feedback signal322 to determine how much delay in the phase of the input voltage signal312 will bring the voltage of the output power signal 316 to thespecified level. For example, the phase controller 310 includes avoltage sense circuit that is configured to sense voltage at 12 voltsrms. If the voltage at the lamps is too high, then the voltage sensecircuit of the phase controller 310 induces a longer delay betweenbefore turning on the power switch 302 with the timer signal 324. Suchvoltage sense circuits and logic rectifier circuits 306 are well knownin the art; however, the use of low power, semiconductor logicrectifiers (e.g., op amps) to full wave rectify the AC power signal 316and the AC lamp return signal 320 for providing a voltage feedbacksignal 322 is novel, regardless of whether the constant voltagecontroller 112 is in the primary or the secondary of the main powertransformer. In triac based controllers, such signals would be rectifiedby full wave bridge rectifiers, which if as described above as are usedin the secondary, experience a large power loss, at least with thecurrent levels in the secondary of the power transformer.

The power switch 302 is preferably embodied as a low loss FET. Since theinput power signal 312 is AC in nature, the low loss switch 302comprises two FETs connected in series. These FETs are speciallydesigned to distribute power signals with low losses since theytypically only have a 0.2 voltage drop or less. Thus, both FETsdissipate about 4 watts with 20 amps of current, in comparison to atriac with a 1.4 voltage drop and a current of about 20 amps (i.e.,about 28 watt power loss in the secondary). Furthermore, FETs are nowknown with about a 0.08 voltage drop, such that two back-to-back FETswould have about a 0.16 voltage drop; thus, a power loss of 3.2 watts(with 20 amps of current in the secondary).

Furthermore, in order to protect against short circuiting of the lamps114, 116 and 118 and other current surges that might damage the powerswitch 302 (or the transformer 104 or the wiring 110) as well as providea soft start for the lamps 114, 116 and 118, the constant voltagecontroller 112 includes a second logic rectifier circuit 308 which isused to provide a current feedback signal 318 to the phase controller310. Thus, the AC output power signal 316 is rectified across a currentsense resistor 334 with the logic rectifier circuit 308, e.g., a fullwave op amp rectifier. Again, the second logic rectifier circuit 308rectifies the AC signal to DC referenced at logic common. The secondlogic rectifier circuit 308 outputs a current feedback signal 318 to thephase controller 310 to delay the turn on of the power switch 302 in theevent of an overload or short circuit to the lamps. Additionally, thecurrent feedback signal 318 advantageously reduces damaging currentsurge to the lamps at power up or switch on. Such a logic rectifiercircuit is well known in the art, although the use of the low powersemiconductor logic rectifier circuit 308 to full wave rectify theoutput power signal 316 to provide a current feedback signal 318 is adeparture from the prior art. Typically, full wave bridge rectifiers areemployed to rectify the AC signal to control a phase controlled triac.However, phase controlled triacs are employed on the primary side of thepower transformer since the current on the primary is much less than thecurrent in the secondary.

Furthermore, a full wave bridge rectifier could be used in the secondaryto convert the AC power signal to a DC power signal such that a controlcircuit in the secondary could easily be designed to control the voltageof the DC power signal to the lamps. However, again, the full wavebridge rectifier suffers from significant power loss in the secondaryand the output power signal to the lamps is a DC signal. In contrast,the input power signal 312 and the output power signal 316 remain as ACsignals in this embodiment. Again, the use of low power semiconductorlogic rectifier circuits, such as op amp rectifiers, have a much lowerpower loss than full wave bridge rectifiers, and result in significantreductions in the power dissipation.

Additionally, in this embodiment, since the current feedback signal 318is provided by the second logic rectifier circuit 308, current sensetransformers, as known in the art, are not required. Such current sensetransformers are expensive and bulky.

It is also noted that the constant voltage controller 112 advantageouslyprovides an AC output power signal 316 that is maintained at the desiredvoltage level irrespective of the length of the wiring run or the numberof lamps coupled to the constant voltage controller 112. The outputpower signal 316 is maintained at this level by providing a reset signal314, the voltage feedback signal 322 and the current feedback signal318, all of which are logic level DC signals that are rectified to DC bylow loss, semiconductor, logic rectifiers, e.g., full wave op amprectifier circuits. This is a departure from the known art in that allof these respective signals are rectified by logic rectifier circuits.Typically, such signals are already in DC (in the event of a DC powersignal) or are rectified to DC by expensive, high loss components (e.g.,current transformers and full wave bridge rectifiers). Thus, the phasecontroller 310, power switch 302, reset controller 304, and the firstand second logic rectifier circuits 306 and 308 are all simple logiccircuits. This “logic only” circuit could be advantageously implementedwithin a single integrated circuit that would be very cost effective incomparison to other control circuits that might use currenttransformers, for example.

Furthermore, it is noted that such a logic only approach of rectifyingan AC voltage power signal to provide a reset signal 314, a currentfeedback signal 318, or a voltage feedback signal 322 is new, regardlessof whether provided at the primary side or secondary side of thetransformer. Power signals of other systems are already in a DC form,since the power signal itself is already in DC, or the signals may berectified to DC using high loss, expensive components, such as currenttransformers or full wave bridge rectifiers.

The result is that the constant voltage controller 112 as a logic onlycontroller can be made very small and easily mounted within a small hubthat measures, for example, 3.5 inches square.

As such, as long as the input power signal 312 is at a voltage that isgreater than the operating voltage level, or desired voltage level ofthe lamps 114, 116 and 118, then the constant voltage controller 112 isable to sense the output voltage and the output current to set the phasecontroller 310 to introduce a desired amount of delay in the phasebefore turning on the power switch 302. Thus, the voltage of the outputpower to the lamps can be regulated to remain at the desired levelregardless of the distance of the lamps from the power transformer, thesupply voltage from the transformer, and whether the number of lamps haschanged.

Additionally, since the voltage feedback signal 322 and the currentfeedback signal 318 are implemented with logic rectifier circuits, i.e.first and second logic rectifier circuits 306 and 308, at the outputpower signal 316 and the lamp return signal 320, additional sensingwires are not required. This is in contrast to conventional triaccontrollers in the primary which require additional sensing wires tofeedback the voltage applied to the lamp loads.

Referring next to FIG. 4, a functional block diagram is shown of anotherembodiment of the constant voltage controller of FIG. 2 employing apulse width modulating controller. Shown is a constant voltagecontroller 400 having the same components as the constant voltagecontroller 112 of FIG. 3, except replacing the phase controller 310 withthe pulse width modulating controller 402 and including a smoothinginductor 406. The remaining components of the constant voltagecontroller 400 of FIG. 4 function the same as those corresponding to theconstant voltage controller 112 of FIG. 3.

The pulse width modulating controller 402, instead of delaying the pointin the phase before the power switch 302 turns on, uses the power switch302 to modulate each cycle of the 60 Hz input power signal 312 at ahigher frequency (see waveform 404), e.g., 2 kHz to 100 kHz, and thensmooth the signal with the smoothing inductor 406 to produce a lowervoltage sinusoidal 60 Hz waveform (see waveform 408), e.g., so that theoutput power signal 316 to the lamps is at the desired voltage level.Such an approach would produce more electromagnetic interference (EMI)and would be more expensive than using a simple phase control circuit.Such pulse width modulating circuits are well known in the art,although, again the use of such circuits in the secondary of the powertransformer is a departure from the known art. Such circuits havepreviously only been applied in the primary side of the powertransformer due to the lower current in the primary. The lower currentof the primary is important due to the power dissipation involved inrectifying the AC signals for the voltage sensing and current sensingcircuits. Again, the use of low power semiconductor logic rectifiercircuits, e.g. op amp rectifiers, to rectify the AC signals to logiclevel DC for the reset signal 314, the current feedback signal 318, thevoltage feedback signal 322, as well as the use of FETs for the powerswitch 302 as opposed to a triac, allows the constant voltage controller112 to be located in the secondary remote from the power transformer.

Referring next to FIG. 5, a schematic diagram 500 of the constantvoltage controller 112 of FIG. 3 is shown in accordance with anembodiment of the invention. The schematic diagram is well understood tothose skilled in the art; thus, no further explanation is provided.However, generally, the various components of the constant voltagecontroller are shown is dashed lines. For example, the power switch 302and the reset controller 304 are illustrated. Also shown are a currentsense circuit 502 (which includes the second logic rectifier circuit308) and a current sense resistor 334. Also, voltage sense circuit 504is shown which includes the first rectifier circuit 306. And a phasecontrol circuit 506 of the phase controller 310 of FIG. 3 is shown. Itis noted that the phase controller 310 of FIG. 3 includes the phasecontrol circuit 506 and various components of the current sense circuit502 and the voltage sense circuit 504 as is understood to one skilled inthe art. Also shown are the comparators 508 and the amplifiers 510.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A power control system comprising: a powertransformer having a primary portion and a secondary portion; a wiringrun having a length and coupled at one end to the secondary portion; aconstant voltage controller coupled to another end of the wiring run,wherein the constant voltage controller receives an input power signalfrom the power transformer and outputs an output power signal at aspecified voltage level to a plurality of low voltage electrical loads,wherein the input power signal is at a higher voltage level than thespecified voltage level, wherein the output power signal is maintainedat the specified voltage level regardless of a change in a voltage dropof the power control system.
 2. The power control system of claim 1wherein the output power signal is maintained at the specified voltagelevel regardless of a change in a voltage drop in the wiring run.
 3. Thepower control system of claim 1 wherein the output power signal ismaintained at the specified voltage level regardless of a change in avoltage drop across the plurality of low voltage electrical loads. 4.The power control system of claim 1 wherein the output power signal ismaintained at the specified voltage regardless of a change in the numberof the plurality of low voltage electrical loads.
 5. The power controlsystem of claim 1 wherein the power transformer comprises a multiple tappower transformer.
 6. The power control system of claim 1 wherein theinput power signal and the output power signal are AC signals.
 7. Thepower control system of claim 6 wherein the constant voltage controllerincludes a reset controller to rectify the input power signal to providea DC reset signal to a controller.
 8. The power control system of claim6 wherein the constant voltage controller includes a first logicrectifier circuit to rectify the output power signal to provide a DCcurrent feedback signal to a controller.
 9. The power control system ofclaim 6 wherein the constant voltage controller includes a first logicrectifier circuit to rectify the output power signal and a lamp returnpower signal to provide a DC current feedback signal to a controller.10. The power control system of claim 9 further comprising: a secondlogic rectifier circuit to rectify the input power signal to provide aDC reset signal to the controller; and a third logic rectifier circuitto rectify the output power signal to provide a DC current feedbacksignal to the controller.
 11. The power control system of claim 1further comprising the plurality of low voltage electrical loads coupledto the constant voltage controller, wherein the plurality of low voltageelectrical loads comprise a plurality of low voltage outdoor lights. 12.The power control system of claim 1 wherein the controller comprises atleast one field effect transistor to switch the input power signal tothe output power signal.
 13. A method of controlling the voltage of apower signal for a plurality of electrical loads comprising: receivingan AC power signal from a secondary portion of a power transformer,wherein the AC power signal is at a voltage greater than a specifiedlevel corresponding to the plurality of electrical loads; adjusting thevoltage of the AC power signal to a voltage level such that the voltagesupplied to the plurality of electrical loads is at the specified level;and maintaining the voltage of the AC power signal at the specifiedlevel regardless of a change in a voltage drop experienced by the ACpower signal.
 14. The method of claim 13 further comprising outputtingthe AC power signal to the plurality of electrical loads.
 15. The methodof claim 14 further comprising rectifying the AC power signal havingbeen output and a return AC power signal from the plurality ofelectrical loads with a first logic rectifier circuit to provide avoltage feedback signal.
 16. A voltage feedback for a power controlsystem for outdoor lighting systems comprising: a switch inputting aninput AC power signal and for providing an AC output power signal to aplurality of lamp loads; a controller coupled to the switch andcontrolling the operation of the switch; and a logic rectifier circuitcoupled to the controller wherein the logic rectifier circuit inputs theAC output power signal and a return AC power signal from the pluralityof lamp loads, wherein the logic rectifier circuit provides a rectifiedvoltage feedback signal for the controller which is used to adjust theoperation of the switch.
 17. The voltage feedback of claim 16 whereinthe logic rectifier circuit comprises a full wave op amp rectifier. 18.The voltage feedback of claim 16 wherein the switch is coupled to aprimary portion of a power transformer and the plurality of lamp loadsare coupled to a secondary portion of the power transformer, wherein theplurality of lamp loads are coupled to the switch via the powertransformer.
 19. The voltage feedback of claim 16 wherein the switch iscoupled to a secondary portion of a power transformer and the pluralityof lamp loads are coupled to the switch.
 20. A current feedback for apower control system for outdoor lighting systems comprising: a switchinputting an input AC power signal and for providing an AC output powersignal to a plurality of lamp loads; a controller coupled to the switchfor controlling the operation of the switch; and a logic rectifiercircuit coupled to the controller wherein the logic rectifier circuitinputs the AC output power signal across a current sense resistor,wherein the logic rectifier circuit provides a rectified currentfeedback signal for the controller which is used to adjust the operationof the switch.
 21. The voltage feedback of claim 20 wherein the logicrectifier comprises a full wave op amp rectifier.
 22. The voltagefeedback of claim 20 wherein the switch is coupled to a primary portionof a power transformer and the plurality of lamp loads are coupled to asecondary portion of the power transformer, wherein the plurality oflamp loads are coupled to the switch via the power transformer.
 23. Thevoltage feedback of claim 20 wherein the switch is coupled to.asecondary portion of a power transformer and the plurality of lamp loadsare coupled to the switch.
 24. A power controller for outdoor lightingsystems comprising: a switch inputting an input AC power signal and forproviding an AC output power signal to a plurality of lamp loads; acontroller coupled to the switch for controlling the operation of theswitch; a logic rectifier circuit coupled to the controller wherein thelogic rectifier circuit inputs the AC input power signal and outputs areset signal for the controller, wherein the reset signal comprises apulse signal sent to the controller at the beginning of each half cycleof the input AC power signal.
 25. The voltage feedback of claim 24wherein the logic rectifier circuit comprises a full wave op amprectifier.
 26. The voltage feedback of claim 24 wherein the switch iscoupled to a primary portion of a power transformer and the plurality oflamp loads are coupled to a secondary portion of the power transformer,wherein the plurality of lamp loads are coupled to the switch via thepower transformer.
 27. The voltage feedback of claim 24 wherein theswitch is coupled to a secondary portion of a power transformer and theplurality of lamp loads are coupled to the switch.